1. FIELD OF THE INVENTION
This invention relates generally to the production of graphic displays of data signals, for example, analog physiological signals such as an electrocardiogram, and relates particularly to sampling type raster scan devices such as display cathode ray tubes or hard copy recorders wherein it is necessary to have a closely spaced raster for good resolution and smooth interconnection of the data points in order that the traces or curves being displayed may give the appearance of continuous lines.
2. DESCRIPTION OF THE PRIOR ART
Graphic display apparatus for providing single or multichannel analog displays and/or records of the amplitude variations of sampled analog data signals are known in the prior art. In one such known form of apparatus, the curves or traces, as plotted on the screen of a cathode ray tube (CRT), comprise a multitude of separated elements or dots instead of continuous lines or traces. Specifically, the plotting of such traces is effected by momentarily unblanking a normally blanked cathode ray beam at points along the successive scan or sweep paths which form the raster, depending upon the amplitudes of the sampled input analog data signals under measurement. Traces of this kind comprising many dots on the screen of the CRT obviously present difficulties in interpretation and analysis.
In another such known form of apparatus, graphic displays are produced from intermittently sampled analog data signals by a so-called connected sampling method that yields substantially continuous, and hence, more discernible curves, notwithstanding the intermittent nature of the sampled data from which the curves are produced. In this form of apparatus, the traces are obtained by producing successive substantially parallel lines on a display medium, for example, a photosensitive record sheet that is arranged to move past the face of a CRT that is operated in a line scan mode. Each of such lines extend between two points on each of the successive sweeps of the cathode ray beam. Between these two points the beam is unblanked. The positions of the points represent, respectively, a corresponding two, consecutively derived ones of the sampled data values. A requirement of this prior art apparatus is that a sampling and comparison step be performed for each sweep and displayed line constituting the trace. An example of a method and apparatus for producing such a connected sampling graphic display from intermittently sampled analog data is shown in U.S. Pat No. 3,605,109 that was issued on Sept. 14, 1971 to Peter R. Lowe and Tommy N. Tyler.
Display apparatus in which the variations in a sampled measured analog data value are converted to digital measured quantities which are stored in a suitable memory before being displayed is also known in the art. Such apparatus provides a display of the variations over a period of time of the analog data value. In such apparatus the sampled measured analog values are stored as individual digital values or data points in an image repeating or recirculating memory. An example of such an apparatus is shown in U.S. Pat. No. 3,653,027 that was issued on Mar. 28, 1972 to David W. Scheer. As there disclosed, an analog-to-digital converter is provided to produce the sampled derived to digital measured quantities. The digital information is recirculated in the memory at a much higher rate than that at which the measured digital values are entered into the memory. Blocks of information are read out of the memory at the memory recirculating frequency. The blocks of information, as read out, are converted back to analog form by a digital to analog converter, and are then presented to the circuit of a cathode ray tube through a comparing means. The cathode ray tube circuit includes a high speed vertical sweep signal and a low speed horizontal sweep signal means. A beam intensity or so-called Z-axis modulation means is activated when the comparing means senses a substantial equality between the analog signal to be displayed and a reference signal related to the high speed sweep signal. Specifically, a normally blanked cathode ray tube beam is unblanked momentarily, when a digital measured value is delivered from the memory, at a position along a vertical sweep or raster line dependent upon the value of the measured value. The representation of the variations in the analog data value appears upon the screen or face of the cathode ray tube as illuminated dots or light points.
A disadvantage of such prior art apparatus is that the dots or light points do not provide smooth and continuous traces or curves. Smoothing out of the traces or curves to make them appear more continuous could be accomplished by providing a raster with more closely spaced lines. While this is a possible solution, it is subject to a serious disadvantage. This is because of the requirement, with the prior art apparatus, even if the aforementioned connected sampling technique were employed, that each raster line must correspond to a data point in the digital memory. That is to say, with the prior art apparatus, the memory must store a data point or value for each raster line. Thus, the larger the number of raster lines that are provided, the larger the memory must be. Thus, the cost of the memory is increased in proportion to the number of raster lines. Since memory is expensive, the provision of more closely spaced raster lines to the known prior art apparatus to achieve smoother and continuous traces or curves has added significantly to the cost of the apparatus.